FIG. 9 is a schematic diagram of a conventional spread spectrum receiver. A transmitter 201 includes a modulation unit 202, a spreading code generator 203, a multiplier 204, a spreading unit 205, a roll-off filter 206, a D/A converter 207, a frequency converting unit 208, and a transmission antenna 209. A receiver 211 includes a frequency converting unit 212, an A/D converter 213, a despreading unit 214, a demodulation unit 215, and a reception antenna 216.
In the transmitter 201, the modulation unit 202 performs a predetermined modulation processing to information to be transmitted and outputs a modulated signal. The multiplier 204 in the spreading unit 205 multiplies the modulated signal by a spreading code that is output from the spreading code generator 203, whereby the spreading unit 205 executes spectrum spreading to the modulated signal. The roll-off filter 206 performs waveform shaping to an output of the spreading unit 205 for band limitation. The D/A converter 207 converts the digital waveform-shaped signal to an analog signal. The frequency converting unit 208 converts frequency of the analog signal to RF signal and outputs a frequency-converted signal by way of the transmission antenna 209.
In the receiver 211, the frequency converting unit 212 converts the signal received via the reception antenna 216 to a baseband signal. The A/D converter 213 converts the analog baseband signal to a digital signal. The despreading unit 214 despreads the digital signal, and the demodulation unit 215 demodulates the despread digital signal.
FIG. 10 is a block diagram of the despreading unit 214. As shown in FIG. 10, a matched filter is often used in the despreading unit. The despreading unit includes a matched filter 221 and an interpolation unit 222. The matched filter 221 performs a correlation operation between the received signal and the reference signal (spreading code) and thereby despreads the received signal. The interpolation unit 222 interpolates a correlation output waveform from the matched filter 221 to thereby improve detection timing accuracy.
According to a sampling theory, if a signal having a band that is limited to W [Hz] is sampled at a sampling frequency that is higher than 2W [Hz], an original signal can be reproduced. If the signal is sampled at a sampling frequency that is lower than 2W [Hz], components at frequencies equal to or higher than W are converted back into a continuous time signal and an original signal cannot be reproduced accurately. This phenomenon is called “aliasing”.
If a chip rate of the spreading code is R [Hz] and a roll-off factor of the roll-off filter 206 is a (0≦α≦1), a frequency band of an output of the roll-off filter is expressed as (1+ α)R/2. In W-CDMA (Wideband-Code Division Multiple Access), for example, α=0.22 is employed.
Therefore, according to the sampling theory, if a sampling rate of the A/D converter 213 is (1+α)R or higher, the original signal can be accurately reproduced. Normally, the sampling rate is set at an integer-multiple of the chip rate of the spreading code to facilitate circuit manufacturing. If characteristic deterioration is to be avoided, a sampling rate of 2R is often used. If circuit miniaturization has a priority, a sampling rate of R is often used.
For determining a sampling frequency of the matched filter circuit, there are following two options:
(1) To satisfy the sampling theory, over-sampling is performed at a frequency that is greater than twice the chip rate of the spreading code;
(2) Deterioration caused by aliasing is acceptable, then the sampling is performed at the same frequency as the chip rate of the spreading code.
If the sampling rate of the A/D converter is set to a frequency that is twice the chip rate of the spreading code, characteristic deterioration does not occur since the rate satisfies the sampling theory. However, since the chip rate is normally high, this sampling rate setting disadvantageously increases circuit scale and power consumption.
If the sampling rate of the A/D converter is set to a frequency that is equal to the chip rate, the circuit scale and power consumption can be reduced; however, aliasing disadvantageously causes the characteristic deterioration.
Attention is now paid to the frequency spectrum of the spread spectrum signal after passing through the roll-off filter. The frequency band of the spread spectrum signal after passing through the roll-off filter (roll-off factor α) is (1+α)R/2. According to the sampling theory, if a signal is sampled at a frequency that is higher than twice the frequency band, an original signal can be accurately reproduced. Therefore, the sampling frequency can be set to R(1+α).
At a=0.22, for example, if a signal is over-sampled at a frequency that is 1.25 times the chip rate, an original signal can be accurately reproduced without characteristic deterioration. Besides, an operation rate of the reception circuit can be set lower than that of two times over-sampling. Nevertheless, because of the odd over-sampling number of 1.25 times the original frequency, the matched filter must have multinary reference signals, which disadvantageously increases the circuit scale. As a result, although the operation rate can be decreased, both the circuit scale and the power consumption disadvantageously increase.